This invention relates to fuel compositions of low sulphur content which contain at least one component capable of reducing particulate emissions from the exhausts of engines which generate power by combustion of such fuels.
Of particular interest are fuels such as diesel which are used rather widely in automotive transport and for providing power for heavy duty equipment such as, e.g., those used in underground mines due to their high fuel economy and low carbon monoxide emissions. However, one of the problems with such fuels is the pollutants in the exhaust gases of diesel powered engines and equipment. For instance, some of the most common pollutants in diesel exhausts are nitric oxide and nitrogen dioxide (hereafter abbreviated as xe2x80x9cNOxxe2x80x9d), hydrocarbons and sulphur dioxide, and to a lesser extent carbon monoxide and carbon dioxide. In addition, diesel powered engines also generate a significant amount of particulate emission which include inter alia soot, adsorbed hydrocarbons and sulphates, which are usually formed due to the incomplete combustion of the fuel and are hence the cause of dense black smoke emitted by such engines through the exhaust. The oxides of sulphur have recently been reduced considerably by refining the fuel, e.g., by hydrodesulphurisation thereby reducing the sulphur levels in the fuel itself and hence in the exhaust emissions. However, the presence of particulate matter in such exhaust emissions has been a more complex problem. It is known that the cause of the particulate matter emission is incomplete combustion of the fuel and to this end attempts have been made to introduce into the fuel organic compounds which have oxygen value therein (hereafter referred to as xe2x80x9coxygenatesxe2x80x9d) to facilitate combustion. Oxygenates are known to facilitate the combustion of fuel to reduce the particulate matter. Examples of such compounds include some of the lower aliphatic esters such as eg the ortho esters of formic and acetic acid, ethers, glycols, polyoxyalkylene glycols, ethers and esters of glycerol, and carbonic acid esters. The following list of references by way of example describe the use of these compounds for reducing particulate emissions and smoke suppression:
a. Society of Automotive Engineers (SAE) Technical Paper Series No. 950400, 1995 by McDonald, J F et al entitled, xe2x80x9cEmissions Characteristics of Soy Methyl Ester Fuels in an IDI Compression Ignition Enginexe2x80x9d.
b. Society of Automotive Engineers (SAE) Technical Paper Series No. 932734, 1993 by Liotta Jr, F J et al entitled, xe2x80x9cThe Effect of Oxygenated Fuels on Emissions from a Modern Heavy-Duty Diesel Enginexe2x80x9d.
c. Society of Automotive Engineers (SAE) Technical Paper Series No. 962115, 1996 by Noboru Miyamoto et al entitled, xe2x80x9cImprovement of Diesel Combustion and Emissions with Addition of Various Oxygenated Agents to Diesel Fuelsxe2x80x9d.
d. EP-A-0 861 882 A1
e. U.S. Pat. No. 5,425,790
f. WO-98 28383
g. WO 98 21293
h. U.S. Pat. No. 5,308,365
i. JP-A-09194859
j. JP-A-07331262
k. U.S. Pat. No. 5,268,008
l. JP-A-62007791
m. JP-A-50058104
Some of the problems with these and other oxygenated compounds are that either
i) they are deficient in other required attributes, eg, boiling point range, cetane number, flash point, toxicity etc. leading to product quality issues; or
ii) they have too much oxygen in them and are hence too polar thereby making them immiscible or incompatible with the fuel; or
iii) they have too little oxygen and hence need to be added in large quantities to achieve the desired improvement in combustion and are hence incompatible with the fuel when used in such large quantities.
It has now been found that these problems can be mitigated by the use of a complex mixture of esters readily generated from commercially available carboxylic acids and hydroxy compounds and which mixture achieves the desired reduction in particulate matter in the exhausts from diesel powered engines.
Accordingly, the present invention is a fuel composition comprising in addition to the fuel at least 3% w/w of an ester additive mixture derivable by reacting together either
(a) (i) a saturated, aliphatic polyhydric alcohol having three or more primary alcohol groups, (ii) a C2-C15 saturated, aliphatic branched chain monohydric alcohol and (iii) a saturated, aliphatic C4-C10 dicarboxylic acid, or
(b) a saturated aliphatic polyhydric alcohol having three or more primary alcohol groups with a C6-C15 saturated, aliphatic straight chain or branched chain monocarboxylic acid, or
(c) a C2-C15 branched chain saturated aliphatic alcohol with a saturated, aliphatic dicarboxylic acid having 6-10 carbon atoms said ester additive mixture having a boiling point of at least 150xc2x0 C. and an oxygen content of at least 13% by weight of said ester additive mixture, the oxygen content being calculated based on atomic weight and molecular structure.
The fuels that may be used in and benefit by the compositions of the present invention comprise inter alia distillate fuels, and typically comprise a major amount of diesel fuel, jet fuel, kerosene or mixtures thereof. The distillate fuel itself may be a conventional petroleum distillate, or may be synthesized, e.g., by the Fischer-Tropsch method or the like. The invention is particularly applicable to diesel fuels.
The ester additive is suitably derived by reacting together either
(a) (i) a saturated, aliphatic polyhydric alcohol having three or more primary alcohol groups, (ii) a C2-C15 saturated, aliphatic, straight or branched chain monohydric alcohol and (iii) a saturated, aliphatic C4-C10 dicarboxylic acid, or
(b) a saturated aliphatic polyhydric alcohol having three or more primary alcohol groups with one or more C6-C15 saturated, aliphatic straight chain or branched chain monocarboxylic acids, or
(c) a C2-C15 branched chain saturated, aliphatic monohydric alcohol with a saturated, aliphatic dicarboxylic acid having 6-10 carbon atoms under esterification conditions.
More specifically, component (i) in ester (a) is suitably selected from one or more of trimethylol ethane, trimethylol propane, monopenta-erythritol, di-pentaerythritol and tri-pentaerythritol. Technical grades of pentaerythritol generally contain a mixture of mono- (88%), di- (10-12%) and the remainder tri-pentaerythritols. Of these, trimethylol propane is preferred.
Component (ii) in ester (a) is suitably selected from one or more of 2-ethyl hexanol, n-octanol, iso-octanol, nonanol, iso-nonanol, decanol, isodecanol, undecanol, dodecanol and isotridecanol. Of these isodecanol is preferred.
Component (iii) in ester (a) is suitably a dicarboxylic acid selected from succinic acid, glutaric acid, adipic acid, sebacic acid, azelaic acid and suberic acid. Of these, adipic acid is preferred.
The esterification of an admixture of components (i), (ii) and (iii) is suitably carried optionally in the presence of an acid catalyst at a temperature in the range from 140xc2x0 C. to about 250xc2x0 C., and a pressure in the range from subatmospheric to atmospheric eg from about 4 KPa to about 105 KPa, over a duration of about 0.1-12 hours, preferably about 2 to 8 hours. The stoichiometry in the reactor is variable, and is capable of vacuum stripping excess reactants, especially acids, to generate a crude ester product which may be further refined by well known methods of contact with compounds capable of removing residual acidity or colour from the product to generate the desired ester mixture. Examples of such compounds capable of removing residual acidity include, activated carbon, alumina, Fuller""s earth, clay, zeolites and the like.
Similarly, in ester (b) the polyhydric alcohol component may be the same as that used for making ester (a). The saturated aliphatic monocarboxylic acid component for ester (b) is suitably a monocarboxylic acid which may be produced by the so called xe2x80x9coxoxe2x80x9d process by hydroformylation of commercial branched C5-C14 olefin fractions to a corresponding branched C6-C15 aldehyde-containing oxonation product. In the process for forming oxo acids, it is desirable to recover the crude oxo-aldehyde intermediate from the oxonation product and then to convert the crude oxo-aldehyde to an oxo acid by oxidation. Acids produced by this oxo process can be linear or branched and usually the branched acids contain methyl groups therein. Examples of such acids include inter alia octanoic acid, iso-octanoic acids, 2-ethyl hexanoic acid, nonanoic acid, iso-nonanoic acids, 3,5,5-trimethyl hexanoic acid, decanoic acid, 2-propyl heptanoic acid and iso-decanoic acids. In this context the expression xe2x80x9cisoxe2x80x9d is meant to convey a multiple isomer product made by the oxo process. In this case, the olefinic feedstream is suitably any C5-C14 olefin, preferably a branched C7 olefin although linear olefins capable of generating linear acids may also be used in the oxo process. The hydroformylation and subsequent oxidation of the crude oxo-aldehyde therefrom can produce linear and branched C6-C15 carboxylic acids, especially a mixture of branched C8 acids which usually comprises eg a mixture of isomers. Particularly preferred branched oxo acids are isooctanoic acid and 3,5,5-trimethylhexanoic acid sold respectively as Cekanoic(copyright)8 acid and Cekanoic(copyright)9 acid commercially by Exxon Chemical Company. Cekanoic(copyright)8 acid typically comprises a mixture of 3,5-dimethyl hexanoic acid, 4,5-dimethyl hexanoic acid, 3,4-dimethyl hexanoic acid, 5-methyl heptanoic acid, 4-methyl heptanoic acid and a mixture of other methyl heptanoic acids and dimethyl hexanoic acids. A mixture of Cekanoic(copyright) 8 and 9 acids can be used in a molar ratio of 1:1 to 1:10, preferably from 1:3 to 1:5 respectively.
The hydroxyl number (measured by an infra-red technique) of a 1:3 mixture is usually of the order of 69 whereas the hydroxyl number (measured by the same infra-red technique) of the 1:5 mixture is about 100-120. These compositions represent partial high hydroxyl esters wherein all of the available hydroxyl groups eg in pentaerythritol are not esterified. To derive the %conversion, the hydroxyl number is divided by 4 and subtracted from 100. Thus in the case of the mixture which contains Cekanoic(copyright) 8 and 9 acids in a ratio of 1:3, the conversion is derived by [100xe2x88x92(69/4)=xcx9c83], i.e., about 83% of the available hydroxyl groups of the pentaerythritol were esterfied in this case. A mixture of esters from which the linear acids C8-C10 monocarboxylic acids can be derived occur in natural oils such as eg in coconut oil. More specifically, such a mixture of linear acids comprising 55% w/w of C8 acids, 40% w/w C10 acids and the remainder being C6 and C12 acids is available commercially from Procter and Gamble. Such mixtures of linear acids may also be used to derive the desired esters of the present invention. A particularly preferred ester is that prepared from a technical grade pentaerythritol and one or both of the Cekanoic(copyright) acids referred to above. This polyol ester can be prepared by mixing Cekanoic(copyright) 8 and 9 acids along with technical grade pentaerythritol in an esterfication reactor and heating it to a maximum temperature of 220xc2x0 C. under an atmosphere of nitrogen. Any water formed in the reaction may be removed from the reaction mixture and then any unreacted acid removed by vacuum stripping. Any residual trace amounts of acids may be neutralized using a dilute sodium carbonate solution followed by flashing water overhead and a final treatment with carbon/clay mixture. The resultant product can then be filtered, e.g., through a solid such as activated carbon, Fullers earth, a zeolite, dicalite, etc., to provide the required product.
Finally in ester (c), the saturated, aliphatic monohydric alcohol having 2-15 carbon atoms is suitably selected from one or more of the group consisting of propanol, isopropanol, secondary butanol, tertiary butanol, the amyl alcohols and the hexanols. As regards the dicarboxylic acid component of the ester (c), it is substantially the same as that used in preparing ester (a) as described above. Ester (c) is preferably diisopropyl adipate.
The ester additive so formed is then admixed with the fuel to be treated in order to reduce the amount of particulate matter emitted by the exhausts of engines powered by the fuel composition.
The ester additive mixture has a boiling point of at least 150xc2x0 C., suitably at least 200xc2x0 C. and preferably above 250xc2x0 C. in order to ensure that it is not too volatile and it does not have a low flash point. For enhancing retention and improving compatibility with the fuel, the ester admixture suitably has an average molecular weight of at least 200, preferably from 200 to 2000 and even more preferably from about 220-1000. In order to improve the potency of the additive in minimizing particulate matter formation, the ester additive mixture should have an oxygen content of at least 13% by weight, suitably at least 15% by weight and preferably more than 20% by weight of said ester additive mixture.
The ester additives most preferred are the esters:
(a) ie that derived from trimethylol propane (1.0 mole), isodecanol (3.03 moles) and adipic acid (2.75 moles) to form a complex alcohol ester (CALE ex Exxon Chemicals) having a viscosity of 165.3 cSt at 40xc2x0 C. and 21.45 cSt at 100xc2x0 C., and having a hydroxyl No. of 18 according to the standard method described in American Oil Chemists Society as A O C S, Cd 13-60; or
(b) ie derived from technical grade pentaerythritol (5 moles), and the Cekanoic(copyright)8 acid (2.5 moles) and 3,5,5-trimethylhexanoic acid (12.5 moles) to form a high hydroxyl polyol ester having a viscosity of 177.8 cSt at 40xc2x0 C. and 13.37 cSt at 100xc2x0 C., and having a hydroxyl No. of 123 according to the standard method described in American Oil Chemists Society as A O C S, Cd 13-60, or
(c) diisopropyl adipate.
The amount of any of the esters (a), (b) or (c) used in the compositions of the present invention is greater than 3% by weight of the total composition, is suitably greater than 5% w/w and is preferably greater than 7% w/w of the total composition. Typically, the esters are used in an amount in the range from 5 to 20% by weight, preferably from 7 to 12% by weight of the total composition. Within these ranges, it would be possible to use a relatively low amount of a specific ester if said ester has a relatively high oxygen content and conversely, one may have to use a higher amount of a particular ester if it is relatively low in oxygen content.
The ester additive used in the fuel compositions of the present invention were evaluated for their performance in reducing particulate emission using a single cylinder Caterpillar 3406 HD engine (which is a Cat 1Y450 engine) with gaseous emission analyses for: hydrocarbons, NOx, carbon monoxide, carbon dioxide, oxygen (Horiba, Mexa-9100 DEGR) and a full dilution particulate tunnel (Horiba, DLS-9200). The particulates generated in the combustion process are collected by placing a filter paper at the exit point of the dilution tunnel from which the exhaust gases emerge. The filter papers used are stabilized and weighed both before and after testing. Stabilization conditions are at a temperature of 20xc2x12xc2x0 C. and at a relative humidity of 45xc2x110%. The difference in weight measured is taken to be the mass of particulate matter collected.